In some radiofrequency information transmission applications, it has been noted that the transmit or receive antenna could have an impedance strongly dependent on conditions outside the antenna, and notably dependent on the environment in which the antenna is located.
In medical telemetry, for example, the antenna may need to be inserted into a probe positioned in the human body, and the impedance then depends strongly on the biological environment in which the antenna is located. It depends on the electrical properties (conductivity, dielectric constant) of the surrounding tissues (muscle, fat) or the fluid environment (blood, other fluids) in which the antenna may be immersed.
Even in more conventional radiofrequency transmission applications (mobile telephony, etc.), the impedance of the antenna may vary.
Generally speaking, antenna impedance variations are particularly sensitive for antennas having very small dimensions and a high quality coefficient, used in applications with substantial miniaturization constraints.
These impedance variations may cause mismatch losses. These losses result from the fact that the transmit channel which feeds the antenna, or the receive channel which receives a signal from the antenna, is generally designed to have optimum performance when it is loaded (at the output for the transmit channel or at the input for the receive channel) by a precisely determined nominal impedance; it has reduced performance when it is loaded by an impedance which differs from its nominal value. Mismatch losses may reach 40 dB.
This is why attempts have already been made to interpose an impedance matching network between the output of the transmit channel and the transmit antenna (and this could also be done at the input for a receive antenna), as a result of which the transmit channel sees an impedance which differs from that of the antenna and is preferably equal to the nominal value for which it was designed, for example 100 ohms or 500 ohms. The matching network is tunable, i.e. its capacitive and/or inductive elements have adjustable values to take account of the environmental conditions of the antenna in such a way that the matching is optimum, regardless of the circumstances.
Document US 2012/0075016 discloses an apparatus and a method for impedance matching in which the matching is obtained by adjusting the impedance values of two elements (for example variable capacitors) as a function of a voltage value measured by a peak detector. A temperature measurement allows the introduction of a correction of the impedance values determined in this way.
Document US 2011/0269416 teaches the use of a temperature measurement combined with information relating to a spectral band in order to perform a preliminary adjustment of the impedances of a matching circuit, followed by a fine adjustment which does not implement a temperature measurement. The preliminary adjustment of the impedances uses a simple correlation table.
In patent application US 2009-0066440, a method has been proposed for automatic impedance matching in a transmit or receive channel in which both the amplitude and phase of the current and voltage at the output of the transmit channel (or at the input of the receive channel) are simultaneously detected. The voltage-to-current ratio is representative of the load impedance Zm seen by the channel loaded by the combination of the matching network and the impedance antenna Zant. The load impedance Zm is measured and the antenna impedance Zant is calculated on the basis of the measured load impedance Zm and the impedances of the matching network, the configuration of which is known at the time of the measurement, and finally the modification that must be applied to one or more of the impedances of the matching network is calculated in order to ensure that the impedance seen by the amplifier becomes matched to the nominal impedance of the amplifier in the current environmental conditions of the antenna.
Document U.S. Pat. No. 4,704,573 discloses a circuit for determining whether the impedance of a load is inside or outside a circle on the Smith chart, characterized by its center and its radius, which may depend on the temperature of an amplifier feeding the load. In this document, as in U.S. Pat. No. 6,965,837, a detection circuit positioned between the amplifier and the load is used to measure the power of the signal at two different locations and deduce therefrom the losses and therefore the better or worse quality of the matching.
These solutions require an electrical contact or an electromagnetic coupling at the output of the amplifier, which may reduce the performance of the latter or the performance of the matching network located downstream of the amplifier.
Generally speaking, mismatch detection circuits themselves influence the matching conditions, which greatly complicates their design. The detection circuit, the matching circuit and the amplifier must be designed at the same time in order to avoid the influence of the detection circuit on the matching.